Creep is the tendency of a material to flow in the solid state. Wikipedia defines it more accurately as deformation http://en.wikipedia.org/wiki/Creep_(deformation).
The rate of deformation is time and temperature dependent. No surprize there. Viscosity, or how resistant a material is to flow dictates how susceptible it is to creeping ( think molasses vis a vis kerosene ).
An example we have all encountered is old pressure sensitive tape. The side of the roll is tacky and the roll exhibits dishing, or "telescoping" if it has had continuous pressure on it. The adhesive has flowed, albeit slowly. Some tapes require a separator sheet made from release liner to keep the sides of the rolls from sticking together.
As creep applies to polyethylene, higher density PE is more rigid and not prone to distortion due to heat and pressure. Linear low density polyethylene ( LLDPE ) is more susceptible to creep than conventional PE especially at lower densities.
Back in the early days of LLDPE, it didn't take long for shipping sack manufacturers to learn that the laws of physics imposed limitations on downgauging, or thickness reduction. The bottom layers of pallets of mulch and soil bags under 3 mils actually ballooned over the side of the pallets, making them inaccessible to forklifts.